Phosgene-free decomposition of dimethylhexane-1,6-dicarbam…
an electron withdrawing character. The C–O group in the O–C=O linkage is
subsequently cleaved yielding an isocyanate product. The resulting surface
methoxide species linked to Zn reacts with the H atom of the surface hydroxyl
species, finally releasing methanol from ZnO.
Conclusion
We have elucidated the method to efficiently decompose HDC to HDI. Catalyst
screening experiments revealed that Zn-containing catalysts Zn(OAc)2 and ZnO
were highly active in the reaction due to the electron withdrawing character of Zn
species. Furthermore, the HDC conversion and HDI yield of the two catalysts were
enhanced using electron-donor PGDE as a solvent. From optimization works over
ZnO selected due to thermal instability of Zn(OAc)2, the reaction condition of
180–200 °C and 2–3 h was appropriate for the highest HDI yield. Additionally,
ZnO was found to be recyclable up to the third use. FT-IR results at elevated
temperatures indicated the removal of the H atom from the N–H group of carbamate
by interaction with the O site of the ZnO surface, the monodentate coordination of
the O–C=O linkage to the Zn site, and the cleavage of the C–O group in O–C=O
linkage yielding the isocyanate product and surface methoxide species that will be
released into methanol from ZnO, thus suggesting a possible reaction scheme.
Although the explanation was developed from results using HDC as the reactant, it
should apply to other aliphatic carbamates since the structural and chemical
characters of ZnO are important factors for decomposition performance.
Acknowledgments This research was supported by the Fusion Research Program for Green
Technologies through the National Research Foundation of Korea (NRF) funded by the Ministry of
Science, ICT & Future Planning (2012M3C1A1054501).
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